This invention relates to a process for producing resinous gravure printing plates which have excellent engraving property or quality, printing durability and solvent resistance.
The processes for producing conventional metal gravure printing plates have required high degrees of skills in electroplating and etching using carbon tissue and also have involved pollution problems. To avoid such difficulties in the conventional processes, resinous gravure printing plates the surfaces of which are composed of resins have been proposed. For example, Japanese Laid-open Patent Applications Nos. 8001/1979 and 22208/1979 disclose processes for forming resinous gravure cells, which comprise coating resins on generally cylindrical substrates to produce printing plate blanks and then engraving the coated resins by an electronic engraver or the like. The resin coating can be applied, for example, by blade-coating wherein solutions of such resins as vinyl chloride resins, ABS resins and polyamide resins are used, or by extrusion or powder-coating wherein the thermoplastic property of resins is utilized.
These conventional processes for producing resinous gravure printing plates, however, are not always satisfactory in connection with the process of forming the resin-coating on the printing subrates to give resin plate blanks and the subsequent mechanical processing. More specifically, the blade coating or the like process utilizes the soluble property of resins, and thus the resulting printing plates have poor resistance to the solvents contained in gravure inks. The extrusion or powder coating process requires a considerably large-scale apparatus and fails to produce such smooth resin surfaces as are required for gravure printing plates. Thus, a surface-smoothing treatment by a super-precision lathe or the like is required after the formation, which results in poor productivity.
Moreover, the above-mentioned resins have low resistance to damage by any processing method, and thus the resulting printing plates lack printing durability. In the course of printing, flaws are apt to be produced on the printing plates by abrasion caused by doctor blades, impurities contained in inks, paper dust, and the like. Thus, almost all of the resulting printing plates are damaged in printing 10 thousand meters or less, and it is difficult to apply these plates to the printing of a large lot of 100 thousand meters or more.
Doctor blades composed of resins have been proposed to overcome such low printing durability. Resinous doctor blades are not practicable because they are inferior in the precision of their blade edges, processability of their blade edges, ink-scraping property, and the like.
Moreover, the above-mentioned resin blanks are inferior in engraving property which is very important for gravure plate blanks. Upon engraving cells on the blanks with a stylus in accordance with copy patterns, burrs and chips are apt to be produced on the peripheries of the cells. The burrs result in printing stains and thus are generally rubbed off with a blade called a burr cutter. However, such rubbing-off operations using burr cutters often fail to remove the burrs completely, and sometimes even push the burrs into the cells.
Accordingly, it is desirable that the resins for gravure printing plates have such engraving property that satisfactory cells are produced without forming burrs and chips and that no burr-cutter processing is required. As described above, the resin blanks used in the conventional processes are not always satisfactory also from the viewpoint of engraving property.
A research group to which we belong has previously proposed an improved process to overcome the above described difficulties in the production of the conventional resin gravure printing plates (cf. Japanese Patent Application No. 95734/1979). In this previously proposed process, a photocurable polyamide resin solution, which is used as a photocurable resin suitable for coating by the knife-coater method and the like, is coated on a cylindrical substrate and cured by irradiation with actinic rays and then gravure cells are formed on the cured film by engraving to produce a resinous gravure printing cylinder.
The present invention is concerned with a further improvement of the above-mentioned process. As a result of our further research, it has been found that the composition of the photocurable polyamide resin solution and especially the ratio of the quantities of the polyamide resin and the polymerizable component such as a photopolymerizable monomeric compound are very important for imparting, to the resulting cured coated film, excellent properties and especially a harmonious balance between engraving property and damage resistance for withstanding abrasion by a doctor blade or the like, although these engraving property and damage resistance seem to conflict with each other. In order to balance the engraving property and the damage resistance, it is especially important that the cured coated film have an ultimate tensile elongation (JIS K 6301) of 100% or less and a tensile strength (JIS K 6301) of 100 Kg/cm.sup.2 or more. This can be achieved only when the quantity of the polyamide resin and that of the polymerizable component such as a photopolymerizable monomeric compound are within the range of restricted ratios. The process for producing gravure printing plates of the present invention is based on these findings.